Latch-Up Consideration (Switch) - Cypress FR81S CY91F52 Series Recommendation For Hardware Setup

Do not connect any input ports directly to VCC or VSS (GND)! Always use pull up or down resistors (2k ... 10k Ohms).
If available, it is possible to use the internal pull up or pull down resistors as well. Please note the value internal resistors
can be 20k ... 100k depends on the device and temperature.
4.2

Latch-up consideration (switch)

Be careful with external switches to VCC or ground together with debouncing capacitors connected to port pins.
A usual configuration is shown in the following schematic:
is a pull-down resistor and CBD a debouncing capacitor. If the switch SW is open, a "0" is read from the port pin
R
PD
Pxy. If the switch is closed the input changes to "1".
From the physical aspect, it has to be considered, that the switch is often placed in distance to the MCU by cable, wire,
or circuit path. The longer the circuit path is the higher will be its inductivity LX (and capacity CX).
An equivalent circuit diagram looks like the following illustration:
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Figure 20. Usual Configuration Switch
Figure 21. Usual Configuration Switch, equivalent circuit
Document No. 002-09375 Rev. *B
Recommendation for Hardware Setup 32-Bit FR81S Family
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